The present invention relates generally to automatic background detection of a scanned document. More particularly, this invention relates to a system and method wherein edge detection parameters corresponding to the color of a backing ski are automatically selected when a backing ski having a first color is to be replaced with a ski of a different color.
In the reproduction of copies of an original document from video image data created, for example, by electronic input scanning from an original document, it is often desirable to provide functions dependent upon determining the exterior edges of the document. Such features include, for example, automatic magnification, automatic two-up copying, deletion of background image data which falls outside the detected exterior edges of the document to avoid storing a document requiring more space than necessary, automatic location of the document in a scanning system, electronic registration and deskewing of the electronic image, etc. In other words, to provide these features, the exterior edges of the document must somehow be detected by the scanning system.
In addition to the features identified above, the identification of the exterior edges of the document is necessary when scanning a document fed with a constant velocity transport (CVT) device, especially in a center-registered document feeding system. Typically, an edge detection operation is used to determine the arrival of individual input documents as well as to determine the side edges of the document fed into the CVT device so as to set the start/stop coordinates in the fast scan direction and thereby identify the image area for capture and processing. That is, the scanner uses the edge detection operation to determine the presence, exact location, and size of a document being imaged in a CVT device.
As should be appreciated, to provide an edge detection operation the exterior edges of the document must somehow be detected by the scanning system. Conventionally, to achieve the detection of the exterior edges of the original document, digital scanners use a backing (e.g., a platen cover or, in a CVT, a baffle or ski) that is readily distinguishable from the original document. That is, edge detection typically relies on the ability of the digital scanner and/or the image processing system to sense a difference, such as the difference in reflectance between the input document""s background and the surrounding backing, e.g., the platen cover, backing plate, baffle, ski, etc. To enable such detection, the document is preferably passed between the scanner and a black (or other dark color) backing. However, backings with a yellow color, a whiter than white color, a backing which fluoresces, and various other alternatives have also been employed.
Although various alternatives have been utilized, it is most desirable to utilize a dark backing when scanning an original document so as to eliminate show through when scanning a double-sided or watermarked document (especially for thin or partially translucent originals). Utilizing a light absorbing backing (e.g., black or dark color) eliminates show through when scanning the document and enables the scanning system or other downstream image processing system to automatically locate the exterior edges of the original document.
However, an undesirable consequence of using a light absorbing backing is that any defect in the original document; such as holes, cuts, rips, dog-ears, etc. or other characteristic properties of the original; such as pre-punched holes, etc.; appear as dark objects (also referred to as a scanning artifact) when they are displayed electronically or rendered and printed on a recording medium, whether they are printed immediately, faxed or subsequently or remotely printed. In the following description, the defects in the original and the characteristic properties of the original will be generally referred to as defects and the corresponding areas in the output document will be referred to as artifacts. However, this notation does not imply that the physical properties of the original are actual defects, just properties that create artifacts in the scanning process which are undesirable.
Various solutions have been proposed to enable accurate edge detection while minimizing show through and removing the scanning artifacts corresponding to holes or similar defects in the scanned image. By having the backing area dark, any area (group of pixels) having a low reflectance value can be considered a potential backing area and thus subject to elimination from the rendered image. However, relying solely on the reflectance value when automatically electronically removing such areas of the scanned-in image is not acceptable since that could also remove actual image data, such as large block printed letter titles, bars, or graphics. Additionally, selectively manually electronically xe2x80x9cpaintingxe2x80x9d over such areas would be too time consuming in most cases, even if available. Given the likelihood of producing many false detections, is desirable to have a second criteria to measure in order to provide proper distinction between backing and the actual image on the document.
Various digital image processing systems for recognizing, and eliminating by image processing, artifacts corresponding to defects in the originals by identifying a finely patterned image on the backing surface have been proposed. Examples of such solutions can be found in U.S. Pat. No. 5,384,621 entitled xe2x80x9cDocument Detection Apparatusxe2x80x9d and U.S. Pat. Nos. 5,959,290; 6,078,051 and 6,122,393 directed to an Image Input Device and Method for Providing Scanning Artifact Detection. Generally, these solutions detect the pattern of a backing by recognizing its electronic signature when it images through a hole in the document sheet and replacing the data with values corresponding to the document background on the assumption that actual text data would not have such a pattern. However, customers that do not normally ever image apertured original documents may not want such a feature implemented, and have no need to even risk any image loss.
Another solution to the above problem is described in U.S. Pat. No. 5,748,344 to Rees. Rees discloses the use of two sets of color sensitive sensors (photosites), each set being sensitive to a different color of light and a backing having a predetermined color. The predetermined color of the backing is selected such that it appears nearly black to one set of sensors and appears nearly white to a different set of sensors. For example, the system might include a backing that is a saturated yellow with a set of sensors that is sensitive to blue being used for edge detection and a set of sensors that is sensitive to green being used for image capture. Such a solution can be used with color scanners wherein a single channel (blue) is used for edge detection and registration with all three sensors (red, green, blue) being used for image capture.
The use of backing having a predetermined color provides an easy way to identify document edges and eliminate dark objects corresponding to document defects from the output document in a black and white scanner. However, this solution also has some disadvantages. For example, in many instances the background of the input document may not necessarily be white, such as when scanning a document printed on colored paper or capturing a pictorial or photographic image. In the case of a non-white document background, it becomes difficult to distinguish the document from the backing. Additionally, the use of a colored backing can adversely impact image quality.
Another proposed solution to reduce the reproduction of document defects as dark objects in the output is disclosed commonly assigned U.S. Pat. No. 6,166,394 entitled xe2x80x9cDual Background Document Scanner to Eliminate Hole Printoutsxe2x80x9d to Robert F. Rubscha. Disclosed therein is a dual mode imaging backing system that includes a substantially dark light absorbing backing surface area and a substantially light reflective backing surface in the same document imaging station as document backing surfaces. The system operates in two modes to automatically provide the dark imaging backing for document edge detection and the light imaging backing to prevent imaging any apertures of any apertured document sheets. The two different imaging backgrounds may be provided by automatically shifting the imaging position between the two adjacent imaging backing surfaces, or by slightly sifting a backing baffle member having the two differently colored areas relative to a fixed position imager. This solution enables the use of a dark backing for most documents with the option to use a light colored backing for eliminating dark objects corresponding to document defects from the output document. However, such a solution increases the complexity and cost of the reproduction system. Additionally, as discussed above, the use of a can adversely impact image quality and not be suitable for the case of a non-white document background.
In accordance with one aspect of the present invention, there is provided an improved document handler and imaging system, wherein different document sheets are sequentially moved past an imaging station by a document feeding system to be illuminated by a document illumination source and imaged by a document imager in the document imaging station, wherein the document imaging station includes a document backing surface having a selected color and a scanning sensor having at least two color sensitive channels and wherein at least one edge of a document sheet is detected in said document imaging station by said document imager, the improvement comprising: a registration parameter detection circuit; the registration parameter detection circuit receiving a set of gray level values for the backing surface from each of the at least two color sensitive channels and automatically determining an average gray level of the backing surface for each of the at least two sensitive channels; the registration parameter detection circuit automatically selecting a registration channel based on the average gray level for each of the at least two sensitive channels and determining a gray level deviation for the registration channel; and the registration parameter detection circuit automatically determining registration parameters based on the average gray level and the gray level deviation of the registration channel.
In accordance with another aspect of the present invention, there is provided a method of automatically detecting registration parameters for a selected backing surface. The method includes (a) scanning the backing surface to obtain at least two sets of gray level values for the backing surface, the two sets of gray level values including a first set of gray level values comprising a gray level value for selected pixel locations along a scanline for a first color channel and a second set of gray level values comprising a gray level value for selected pixels locations along a scanline for a second color channel; (b) determining an average gray level for each of the two color channels; (c) selecting a registration channel based on the average gray level; (d) determining a gray level deviation for the registration channel; and (e) determining registration parameters based on the average gray level and the gray level deviation of the registration channel.
In accordance with another aspect of the present invention, there is provided an improved document handler and imaging system, wherein document sheets are moved past an imaging station by a document feeding system including rollers coupled to a shaft to be illuminated by a document illumination source and imaged by a document imager in the document imaging station, wherein the document imaging station includes a scanning sensor having at least two color sensitive channels and wherein at least one edge of a document sheet is detected in said document imaging station by said document imager. The improved document handler and imaging system including a detachable backing surface connected to the shaft in a manner to permit the shaft to freely rotate; and a registration parameter detection circuit. The registration parameter detection circuit receiving a set of gray level values for the backing surface from each of the at least two color sensitive channels and automatically determining an average gray level of the backing surface for each of the at least two sensitive channels; selecting a registration channel based on the average gray level for each of the at least two sensitive channels and determining a gray level deviation for the registration channel; and determining registration parameters based on the average gray level and the gray level deviation of the registration channel.